This invention relates generally to a device for producing a frequency standard, and, more particularly, to a small, rugged and relatively inexpensive laser for providing a stable frequency standard for use within a clock.
Most clocks, and, in particular, clocks which are extremely accurate and precise are based, in their operation, on frequency standards. For periodic events, the time between the events, t, is related to the frequency, v, of their occurrence by the simple equation v=1/t. Periodic events can be used to define time, i.e., the generator of the periodic events -- the frequency standard -- can be used as a clock. The frequency standard becomes a clock by the addition of a counting mechanism for the events.
The first clocks based on a frequency standard (a pendulum) were invented about 400 years ago. This type of clock is still most widely used today. The pendulum may be a suspended weight (gravitational pendulum) like in "grandfather" clocks or the balance (torsion pendulum) of modern wristwatches. The instant invention deals with today's most advanced frequency standards and clocks; however, a close look at traditional clocks show all the essential features which are utilized in quartz crystal and atomic clocks.
The unit of time today is the second (symbol s). The second is defined in reference to a frequency determining element. Since 1967 by international agreement this "natural pendulum" is the cesium atom. One second is defined in the official wording as "the duration of 9192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom". Accordingly, the frequency of the cesium pendulum is 9192 631 770 events per second (the cesium atom is a very rapidly oscillating pendulum). The unit of frequency is then defined as hertz (symbol Hz) which means the repetitive occurrence of one event per second (the use of "hertz" is preferred to the older term "cycle per second", cps).
Many kinds of frequency determining elements have been and are being used in frequency standards. They can be grouped into three classes: mechanical resonators; electronic resonators; atomic resonators.
As far as mechanical resonators are concerned most accurate clocks deal only with the quartz crystals. Other mechanical resonators like the pendulum and the tuning fork are of no importance in today's high performance frequency standards although they have been historically very important and are still widely used in low performance devices (e.g., in watches). For similar reasons electronic resonators like the tank circuits are unable to provide an adequate frequency standard for high precision clocks. Atomic resonators form the heart of our most accurate frequency standards and clocks.
Unfortunately, the atomic resonators such as the Cesium Beam Frequency Standard, Rubidium Gas Cell Frequency Standard and Atomic Hydrogen Maser leave much to be desired when it comes to the production of a small, lightweight, rugged, inexpensive high performance clock. Frequency standards based on locking a laser to a low pressure absorption feature have also produced excellent characteristics. However, this type of frequency standard requires large, awkward and expensive multiplier chains to translate their frequency into a usable region.